In a digital signal processing circuit the later gain stages are typically alternating current (AC) coupled to maximize the signal dynamic range at the analog to digital converter (ADC). There are situations where the direct current (DC) signal level after the first gain stage needs to be evaluated. If that DC bias is allowed to reach the limits of the circuit (voltage rail) it will result in signal corruption and the AC response will not be valid. Typically gain transients are not useful since the circuit is settling from one quiescent state to another and need to be ignored. Using gain characteristics of the first gain stage allows a transition from high to low gain to generate a bias change (known transient behavior). This transition is seen as a step response to the AC circuit and the AC response can be evaluated to determine the bias present at the input to the AC coupled circuitry (verify no signal corruption).
What is needed, therefore, are techniques for determining that the input bias levels are not close to the voltage rails at the input to an AC coupled system to avoid signal corruption. Typical solutions involve the addition of monitoring circuitry that measures the DC bias directly. This requires additional circuitry and ADC inputs which are not cost effective in a product that is already in production or applications that have limited board area.